1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Conventional wireless communication systems include one or more base stations, which may also be referred to as node-Bs, for providing wireless connectivity to one or more mobile units, which may also be referred to using terms such as user equipment, subscriber equipment, and access terminals. Exemplary mobile units include cellular telephones, personal data assistants, smart phones, text messaging devices, laptop computers, desktop computers, and the like. Each base station may provide wireless connectivity to one or more mobile units in a geographical area, or cell, associated with the base station. For example, a base station that operates according to a Universal Mobile Telecommunication System (UMTS) protocol may provide wireless connectivity to one or more mobile units in a cell associated with the base station over a wireless communication link.
One technique that permits each base station to transmit concurrently to multiple mobile units is orthogonal frequency-division multiplexing (OFDM). In OFDM, a single transmitter (e.g., a base station) may transmit signals on many different orthogonal frequencies, which may also be referred to as sub-carriers or tones. For example, an OFDM carrier signal may include dozens to thousands of orthogonal sub-carriers. The baseband data carried over each sub-carrier may be independently modulated, e.g., using quadrature amplitude modulation (QAM), phase-shift keying (PSK), or some other modulation scheme. For example, the modulation scheme for a given sub-carrier may be determined based on channel quality information associated with the sub-carrier. The composite baseband signal is typically used to modulate a main radiofrequency carrier wave. Each mobile unit may receive information from the base station via one or more sub-carriers.
Downlink radio resources in both the temporal domain and the frequency domain may be controlled by a scheduler in the base station. Conventional scheduling algorithms associate a priority with each mobile unit to indicate the order in which the scheduling algorithm should consider assigning one or more subcarriers to the mobile unit. Resources may then be allocated to the mobile units in the order indicated by the priority. Resource allocation typically includes determining powers and/or bandwidth to optimize performance within the cell served by the base station. For example, the power allocated to each subcarrier may be determined using a “water filling” algorithm. In the water filling algorithm, the channel gain associated with each subcarrier is inverted to form a series of “peaks” and “valleys” in a graphical representation of the inverted channel gain. A water level is selected for the graphical representation so that the difference between the inverted channel gain of each channel and the water level is proportional to the power allocated to the channel, subject to the constraint that the sum of the allocated powers is less than or equal to the total available transmission power.
Interference mitigation schemes may also be implemented to reduce interference between the cell served by the base station and adjacent cells. For example, base stations in adjacent cells may transmit using the same set of frequency channels. If both base stations allocate the same frequency channel to different mobile units, then the mobile units may receive a composite signal including signals from both base stations on the assigned frequency channel. One portion of the composite signal is the desired signal and another portion of the composite signal will be seen as interference, which is conventionally referred to as “co-channel interference.” Co-channel interference may interfere with, and potentially disrupt, communication with the mobile units. Thus, interference mitigation schemes typically attempt to minimize co-channel interference by coordinating assignment of the radio channels, and the power assigned to each channel, among base stations that serve adjacent cells. For example, mobile units near the edge of a cell are typically assigned to a subset of the frequency channels that may transmit at a higher power. Conventional interference mitigation schemes attempt to reduce co-channel interference caused by these higher power transmissions by coordinating frequency channel assignment so that base stations serving adjacent cells use different subsets of the frequency channels for the higher power transmissions.
However, conventional resource allocation schemes such as the water filling algorithm described above generally operate independently of interference mitigation schemes. Consequently, the conventional resource allocation schemes may not optimally allocate resources. To the contrary, in some cases the resource allocation schemes and the interference mitigation schemes may attempt to allocate resources in a contradictory manner. For example, the water filling algorithm typically allocates more power to channels that have a low channel gain, i.e., channels that have the best channel quality. However, channels that are used to transmit to mobile units near the edge of a cell typically have a relatively low channel quality and a correspondingly high channel loss. Accordingly, conventional interference mitigation schemes may attempt to assign a relatively high power level to mobile units in the protected regions, whereas the conventional resource allocation scheme may attempt to assign a relatively low power level to the same mobile units.